1. Field of the invention
The present invention relates to a member of an analytical element which is advantageously employable in the dry system analysis of a liquid sample containing solid material.
2. Description of prior arts
There are known a number of analytical systems for detecting and analyzing biochemical active components present in liquid samples by using a dry analytical element in a layer form (sheet form) (see, U.S. Pat. No. 3,050,373, etc.). Generally, the analysis of the liquid sample in these analytical systems is conducted in such a manner that a reactive component capable of physically or chemically reacting with a substance to be analyzed (an analyte) contained in the liquid sample, is previously incorporated into the analytical element, the reaction of said analyte introduced into the element with said reactive component is allowed to proceed in a biochemical reactive layer and the amount of the reaction product or unreacted component is determined through spectrophotometry or fluorimetry or using isotope radioactivity, thus assaying the analyte.
The above-mentioned dry analytical method has been widely used in the fields of immunological assay utilizing an antigen-antibody reaction or the analysis of enzyme or substrate using an enzyme reaction, since analytical operation is relatively simple. While the analytical method using the analytical element is advantageously simple, there have been made attempts of finding out the layer structure of the analytical element which can give such a wide latitude that the element can be applicable to the analysis of various liquid samples. For example, it has been proposed that in the case of using a liquid containing solid material, particularly whole blood, as a test sample, a blood cell-filtering layer be provided above a reagent layer to filter erythrocyte (i.e., red blood cell). A typical example of such a blood cell-filtering layer is disclosed in Japanese Patent Publication No. 53(1978)-21677 wherein the filtration of blood cells is carried out using a material having a proper porosity. This patent specification teaches that the pore size of the filtering layer should be in the range of 1 to 5 .mu.m which is smaller than that of the blood cell whose size is in the range of 7 to 30 .mu.m. Namely, the blood cells can be separated from the liquid phase such as serum and plasma by subjecting the liquid sample to surface filtration in which the blood cells can not penetrate into the filtering layer, but can remain unfiltered on the surface thereof. The blood cell separation by the surface filtration using an analytical element with a built-in blood cell-filtering layer is relatively simple as compared with conventional methods wherein whole blood is subjected to centrifugation. However, the filtering rate is not so high and the filtration layer is liable to be clogged. Consequently, the liquid sample poorly spreads, whereby the analytical sensitivity is lowered with analytical accuracy is reduced.
There is disclosed in Japanese Patent Provisional Publication No. 57(1982)-53661 an element for removing the solid material from blood by separation of plasma and serum by passing it through a layer composed of a specific glass fiber having an average diameter of 0.2 to 5 .mu.m and a density of 0.1 to 0.5 g./cm.sup.3. However, this element is not considered to be fully satisfactory with the blood cell separation ability. According to the description of the examples thereof, the practical separation of plasma (or serum) and blood cell is performed under limitation of the amount of serum or plasma to be applied to a multilayer analytical element in the analytical operation to 50% or less of the absorption amount of the layer and in the presence of a hydrophobic barrier layer. The above separation element has been proposed based on the understanding that serum or plasma passes through the glass fiber at a rate higher than that of blood cell. Thus, the solid-liquid separation on the basis of the volume filtration of the present invention is not suggested.